Subsurface Utility Engineering and the Quality Levels of Mapping

Everyone has either been involved in a project where unexpected utilities wreak havoc on the site, or at the very least has heard such war stories.  The impact of these unexpected utilities can be tremendous and include cost overruns, schedule delays, and even health and safety impacts. 

There are ways to help mitigate risk regarding utilities. Subsurface Utility Engineering (SUE) and the associated Quality Levels of Mapping is a great way to put the project team in control of the role subsurface utilities will play on your project. For starters, Subsurface Utility Engineering is a hybrid land surveying and professional engineering service to map, designate, and locate existing utilities; determine and resolve potential conflicts; design relocations of utilities or alternate project layouts; create construction plans, specifications, and estimates for utility-related work, and negotiate and finalize shared agreements between all parties. Understanding SUE and the Quality Levels of Mapping and having this information up front, in the design phase, before the job goes out to bid and into construction, puts the engineer and owner in the driver’s seat. Identifying potential issues in the design phase before the construction creates a more cost-effective and safer project.

Scanning for the target utility with the receiver

Prior to a standard being put in place, utilities would be drafted on a drawing in many different fashions, and the user may or may not have had an understanding as to how the information was derived. Through the 1980’s and 1990’s, DOTs throughout the country refined a system of quality levels of utility mapping.  In 2002, the American Society of Civil Engineers sought to standardize this process and published ASCE 38-02, Standard Guideline for the Collection and Depiction of Existing Subsurface Utility Data. This has been recently updated to ASCE 38-22. The Quality Levels of Mapping are defined in these guidelines and identified in 4 quality levels. These quality levels are D, C, B, and A. 

In a Quality Level D utility map, the lowest of the 4 levels, utilities are traced on a drawing by records and oral recollections only.  In a Quality Level C utility map, utility information derived during D is upgraded through a ground survey of utility hardware on the property. The drafter can then more accurately map the utilities through the surveyed infrastructure. In a Quality Level B utility map, utility record information acquired in quality levels D and C are designated in the field by a qualified SUE technician utilizing different geophysical methods. These methods include electromagnetic induction (EMI) and ground penetrating radar (GPR). The utilities are designated (traced out) and paint marks and flags are placed on the ground identifying the horizontal location of each underground utility. A surveyor then performs a ground survey of the underground utility hardware, and the utility paint marks and flags. This enables the drafter to draft the utilities with higher horizontal or positional accuracy.

Quality Level B layout

When vertical information is required on a project, Quality Level A is deployed.  On a Quality Level A utility map, utilities are exposed, either by hand or vacuum excavation and surveyed beneath the surface. Exposed utility information is confirmed regarding pipe size, elevation, material, and condition. Quality Level A gives you the precise horizontal and vertical location of the utility. 

It is important to keep in mind that all quality levels have value, and all levels can be used on the same drawing. The appropriate quality level necessary is based on the design requirements for the area of the project. Quality Level B might be appropriate for most of a drawing. Quality Level A is appropriate in a site-specific area where the vertical information is critical for design. For example, if a gravity sewer line is being designed, the path of the proposed sewer line would be the area on the drawing where a recommendation to improve from Quality Level B to Quality Level A would be appropriate. This will enable the identification, elevation, material, and condition of the crossing utility lines to be identified. With this information, the engineer can design the elevation of the sewer line based on the utilities crossing and the elevation of the structure connecting to.  

Placing paint marks

SUE and the Quality Levels of Mapping have proven to provide cost savings for projects. In 1999, Purdue University performed the benchmark study. They were commissioned by the Federal Highway Administration (FHWA) to analyze and quantify the cost savings of SUE on 71 highway projects. Purdue University concluded that a cost savings of $4.62 was derived for every $1.00 spent on SUE. Since then, many other studies have been performed analyzing the cost savings of using SUE on projects. In 2005, a study was done in Toronto on nine projects, yielding a savings between $2.05 and $6.59. In 2007, Penn State University did a study analyzing 10 projects, yielding a cost savings of $22.21. In 2021, the Louisiana Study was done yielding $2.73 savings on three projects for every dollar spent on SUE. Different types of jobs and different cost-saving analyses were some of the reasons for the differing cost-saving discrepancies. What is important to note is that after all the studies were complete, there was a cost-saving for performing SUE. Using the Purdue model, if you spend $10,000 in SUE fees on a project, the savings would be $46,200. If you spent $20,000 on SUE, your cost saving would be $92,400.

The bottom line is that SUE and the Quality Levels of Mapping are a proven value when used in the design phase of a project – they put the owner and engineer in control of their project, as construction change orders and schedule delays are minimized, as well as the many other less measurable impacts associated with unknown utilities.

Please reach out to me to find out how SUE can help your next project: rbracco@blcompanies.com.